In this description and claims, the term “LED” will be used to denote both organic and inorganic LEDs, and the invention can be applied to both categories. The detailed examples below are based on OLEDs but all examples can use inorganic LEDs instead.
LEDs are current driven lighting units. They are driven using an LED driver which delivers a desired current to the LED.
The required current to be supplied varies for different lighting units, and for different configurations of lighting unit. The latest LED drivers are designed to have sufficient flexibility that they can be used for a wide range of different lighting units, and for a range of numbers of lighting units.
To enable this flexibility, it is known for the driver to operate within a so-called “operating window”. An operating window defines a relationship between the output voltage and output current than can be delivered by the driver. Providing the requirements of a particular lighting load fall within this operating window, the driver is able to be configured for use with that particular lighting load, giving the desired driver flexibility.
The driver has its output current set to the desired level within its operating window. This can be achieved by programming the driver to deliver a specific current or by providing current setting information using an input to the driver. This input can be connected to a setting resistor or other component, outside the driver, which is read by the driver. The value of the current setting resistor or other component is measured by the driver, which can then configure its output accordingly, so that the output current is determined by the resistance value. The important point is that after the driver has been produced, the output current can be selected, so that a single driver design is suitable for a range of output currents.
Once the current has been set, the voltage delivered by the driver will vary depending on the load presented to it (since the LEDs are current driven), but the driver will maintain this voltage within the operating window.
LEDs can fail in several ways. One way an OLED can fail is a defect within the organic stack, where the current does not flow in a well-distributed path over the whole lighting surface, but through a single point. This defect is a so-called short. Instead of a typical electrical short, where the resistance is somewhere around 0Ω, the defect of an organic stack behaves like a low ohm resistance. Depending on the used current, a high power consumption may appear which is locally converted into heat within the short point. Therefore a short circuit protection is necessary to keep the short point temperature below a temperature level (e.g. 70° C. surface temperature) or minimize other risks from short circuits as required by safety standards, and to prevent excessive power consumption, where more heat but less light is generated by the OLED.
Typically, the normal operation voltage of an OLED at a given current level is much higher than the voltage which appears in case of a short. Therefore, a short protection can be easily realized by using a comparator circuit which will switch off/bypass the OLED if the voltage is below a defined voltage threshold.
Using a simple threshold test is a crude way to detect a short condition which can result in false detection or it can require the operating range to be compromised. There is therefore a need for an improved approach to the detection of a short condition.